1. Field of the Invention
This invention relates to a toner to be used for electrophotography, electrostatic printing, magnetic recording or electrostatic printing, particularly to a toner suitable for pressure fixing.
2. Description of the Prior Art
The operation of the so called "fixing" has heretofore been practiced when electrostatic latent image or magnetic latent image is desired to be developed with colored powder called "toner" and to store the resultant image thereafter. As such fixing methods are known a method in which the toner image on the latent image bearing member surface is fixed or attached as such, or after transferred onto a transfer member, by melting the toner in a heat chamber, a method in which the toner is pressure bonded onto a support simultaneously with melting by means of hot rollers, and a method in which toner is attached by dissolving it in a solvent and thereafter removing the solvent.
When heat is used as described above, for utillizing more effectively the heat, it is sometimes practiced to use flash exposure, IR-ray exposure, induction heating or a combination of these. However, there is a limit to substantial improvement in efficiency, and the heat quantity required per sheet is the same. Accordingly, when fixing is desired to be performed at a higher speed, there is no way but to supply greater amount of heat. In these days, when techniques are highly progressed, and image information is also required to be transmitted at high speed with diversification and speed-up of information, the fixing time for storage of image information is demanded to be very short. In compliance with such a demand, under the present situation, a fixing device having a gigantic heating source is employed.
As contrasted to the fixing methods known in the art as described above, as a fixing method attracting attention recently, there is a fixing method called the pressure fixing method in which image is fixed only by pressure. This method enables fixing only by pressure and therefore has an advantage of obviating the operation to give heat as required in other methods of the prior art, but its still greater advantage is "requiring little time for fixing". That is, pressure fixing is effected momentarily when the image passes through the pressure rollers. In the pressure fixing, fixing is effected by only pressure, and such a time consuming step as melting or vaporization is not required at all. It may well be natural to consider that the pressure fixing method will be promising as a means for higher speed fixing, for the reasons as mentioned above.
However, in spite of great advantages as mentioned above, the pressure fixing method known in the art involves some vital drawbacks. One of them is the pressure required for fixing, which is generally 20 to 75 kg/cm in terms of line pressure. For application of such a force, the fixing device is required to have a considerable strength, to make the fixing device undesirably large and heavy. Another drawback is that it is extremely and essentially difficult to apply a pressure as mentioned above evenly on an image on a transfer paper, whereby it is very difficult to prevent the transfer paper from creasing or curling. Further, when a pressure as mentioned above is applied on the image by rollers, the image surface will be flattened to give rise to lustre on the image and lower the quality of image. Also, since the fixing characteristic depends greatly on the line pressure and the properties of the fixing substrate on which toner is fixed, the fixing characteristic may be changed if, for example, the width of the paper is changed or the transfer paper is changed in the same fixing device. This is also a serious drawback experienced as a trouble in practical application. Actually, it has been attempted to circumvent these problems in one way or another, and some of them are actually put into practice. For example, for removing lustre, fixing rollers are made matte, or for application of uniform pressure, a crossing angle is provided between the rollers. However, even by recourse to such methods, other problems are caused instead. For example, the matte roller itself will be returned to a flat roller due to large fixing pressure, or crease tends to develop more readily due to the crossing angle.